* ‘Die Näturlichen Einrichtungen zum Schutze
des Chlorophylls,’ etc., 1876. Pringsheim
has recently observed under the microscope the destruction
of chlorophyll in a few minutes by the action of concentrated
light from the sun, in the presence of oxygen.
See, also, Stahl on the protection of chlorophyll
from intense light, in ‘Bot. Zeitung,’
1880. [page 447]

or twist laterally, so as to place their edges in
the direction of the sun’s light; but Cohn long
ago observed that the leaflets of Oxalis bend downwards
when fully exposed to the sun. We witnessed a
striking instance of this movement in the very large
leaflets of O. Ortegesii. A similar movement
may frequently be observed with the leaflets of Averrhoa
bilimbi (a member of the Oxalidae); and a leaf is
here represented (Fig. 180) on which the sun had shone.
A diagram (Fig. 134) was given in the last chapter,
representing the oscillations by which a leaflet rapidly
descended under these circumstances; and the movement
may be seen closely to resemble that (Fig. 133) by

Fig. 180. Averrhoa bilimbi: leaf with leaflets
depressed after exposure to sunshine; but the leaflets
are sometimes more depressed than is here shown.
Figure much reduced.

which it assumed its nocturnal position. It is
an interesting fact in relation to our present subject
that, as Prof. Batalin informs us in a letter,
dated February, 1879, the leaflets of Oxalis acetosella
may be daily exposed to the sun during many weeks,
and they do not suffer if they are allowed to depress
themselves; but if this be prevented, they lose their
colour and wither in two or three days. Yet the
duration of a leaf is about two months, when subjected
only to diffused light; and in this case the leaflets
never sink downwards during the day.]

As the upward movements of the leaflets of Robinia,
and the downward movements of those of Oxalis, have
been proved to be highly beneficial to these plants
when subjected to bright sunshine, it seems probable
that they have been acquired for the special purpose
of avoiding too intense an illumination. As it
would have been very troublesome in all the above cases
to [page 448] have watched for a fitting opportunity
and to have traced the movement of the leaves whilst
they were fully exposed to the sunshine, we did not
ascertain whether paraheliotropism always consisted
of modified circumnutation; but this certainly was
the case with the Averrhoa, and probably with the
other species, as their leaves were continually circumnutating.
[page 449]